The present invention relates to Device-to-Device (D2D) Communications in the Cellular Spectrum. Although the idea of enabling D2D communications as a means of relaying in cellular networks was proposed by some early works on ad hoc networks, the concept of allowing local D2D communications to (re)use cellular spectrum resources simultaneously with ongoing cellular traffic is relatively new. Because the non-orthogonal resource sharing between the cellular and the D2D layers has the potential of the reuse gain and proximity gain at the same time increasing the resource utilization, D2D communications underlying cellular networks has received considerable interest in the recent years.
Specifically, in 3GPP LTE networks, such LTE Direct (D2D) communication can be used in commercial applications, such as cellular network offloading, proximity based social networking, or in public safety situations in which first responders need to communicate with each other and with people in the disaster area.
D2D communication entities using an LTE Direct link may reuse the same physical resource blocks (PRB) as used for cellular communications either in the downlink or in the uplink or both. The reuse of radio resources in a controlled fashion can lead to the increase of spectral efficiency at the expense of some increase of the intra-cell interference. Typically, D2D communicating entities use UL resources such as UL PRBs or UL time slots, but conceptually it is possible that D2D (LTE Direct) communications takes place in the cellular DL spectrum or in DL time slots. For ease of presentation, in the present disclosure we assume that D2D links use uplink resources, such as uplink PRBs in an FDD or uplink time slots in an a cellular TDD system, but the main ideas would carry over to cases in which D2D communications take place in DL spectrum as well.
Simultaneous D2D and Cellular Transmissions in D2D Communications
FIG. 3 shows a principal schematic sketch over a network assisted D2D system. One or more network nodes 310 are in control over at least one radio frequency communication device 320, 325 and 327 (also referenced D1, D2 and D3), of which at least two (320 D1 and 325 D2) are also is involved with D2D communication with each other. The network node 310 allocates time-frequency resources for D2D transmission, and is also in control over maximum allowed transmission (TX) power used in the D2D communication. In a typical scenario, the network node 310 allocates D2D resources for approximately 200-500 ms and during that time period, then each radio frequency communication device 320, 325 makes autonomous selections of MCS (modulation and coding scheme) and executes commands such as HARQ (hybrid automatic repeat request). At the end of each time period, the radio frequency communication device 320 reports signal quality status and/or other transmission quality measures, and receives new D2D resources to use for the next time period (i.e. 200-500 ms).
Furthermore, typically UpLink (UL) spectrum/resources are used for D2D, as this is beneficial from an interference control perspective. And, as D2D communication will typically not take up too much of the spectrum resources into account, it is far from efficient to allocate an entire frequency bandwidth in a sub frame for D2D communication. Hence, both UL (and Downlink (DL)) traffic and D2D traffic need to able to share the same sub frames, for example sharing a frequency. This also means that, for optimized spectrum usage, a first radio frequency communication device might be able to transmit to the network node 310 and to a second radio frequency communication device 325 in the same sub frame.
FIG. 5 shows an example of how simultaneous cellular and D2D allocation in the UL can be made. In time block or time period A, the first radio frequency communication device 325 D1 transmits D2D and simultaneously a physical UL control channel (PUCCH) to the network node 310. In time period B, the second radio frequency communication device 325 D2 transmits a physical UL shared channel (PUSCH) to the network node 310 and simultaneously to the first radio frequency communication device 320 D1. In time period C, both the first radio frequency communication device 320 D1 and the third radio frequency communication device 327 D3 transmit a PUSCH respectively to the network node 310. In time period D, the second radio frequency communication device 325 D2 transmits a physical UL shared channel (PUSCH) to the network node 310, while in time period E, the second radio frequency communication device 325 D2 transmit to the first radio frequency communication device 325 D1 in D2D and the third radio frequency communication device 327 D3 transmit a PUSCH to the network node 310.
The performance of multi-channel radio systems in general, and orthogonal frequency division multiplexing (OFDM) systems in particular, can be severely impacted by the interference due to concurrent transmissions on adjacent channels. This is because radiation or “leakage” of energy from adjacent frequency channels can cause significant interference. This ACI can be more severe when the transmission power level on the adjacent channel is high, especially when there is a large imbalance between the transmission levels on adjacent channels. For example, in 3GPP the Adjacent Channel Leakage Power ration (ACLR) is an example of a measurement that can be used to characterize the leakage power problem.
Problems with Existing Solutions
The scheduling flexibility requirement desired and discussed above, (namely that a device simultaneously should be able to transmit to a Network node 310 and to another device in the same sub frame) gives rise to the following problems.
One problem is that the total transmission power may not be sufficient for maintaining simultaneous transmission with sufficient quality. Since the radio frequency communication devices of a D2D pair are typically in close proximity to each other, a low transmission power is typically needed in the D2D communication. However, the radio frequency communication device might be far away from the network node 310 and therefore it might need high transmission power for transmission to the network node 310. In fact, D2D communication reusing cellular PRBs is typically expected to take place between radio frequency communication devices that are close to one another but sufficiently far from a network node such as a base station (eNB). Another problem lies in that since the device transmitter or radio frequency communication device's radio frequency interface, especially the power amplifier (PA), is not ideal, due to non-linearities; the transmission on a first set of Resource Blocks (RBs) gives rise to spectral emission on adjacent RBs within the system frequency band.
Another problem lies in that the transmission power imbalance, which is the difference (in logarithmic scale) between the powers transmitted by a radio frequency communication device simultaneously on the D2D transmission and the Network node 310 transmission links, might become too large. This can occur when, for example, the first radio frequency communication device is far from its serving network node but closer to a second radio frequency communication device which the first radio frequency communication device is involved in D2D communication with. This may also occur when, for example, the data rate on the two links differ significantly. A large transmission power imbalance degrades the performance of the weaker link due to leakage of signal transmitted on the stronger link. The radio frequency communication device can handle up to a certain transmission power imbalance.
FIGS. 6A and 6B illustrate a problem that may arise due to this in-band emission. In FIG. 6A, showing a time period A (referring to time period A in FIG. 5), the first radio frequency communication device 320 D1 transmits a PUCCH to the network node 310 (referenced D1→NW) and at the same time also in D2D mode to the second radio frequency communication device 325 D2 (referenced D1→D2). Assuming the PUCCH is transmitted with higher power than the D2D part (due to e.g. path loss differences or different SINR (Signal-to-Noise-and-Interference-Ratio) targets), the D2D transmission may be affected by transmission leakage (referenced LA) from the PUCCH transmission. However, in this case the imbalance is not too large and hence transmission leakage will not significantly affect the D2D transmission. This is exemplified in the D2D signal constellation (below in FIG. 6A) (assuming a QPSK signal is transmitted on one sub-carrier).
However, in FIG. 6B, showing a time period B (referring to time period B in FIG. 5), where D2 is transmitting a PUSCH (referenced D2→NW) at the same time as D2D-communicating with D1 (referenced D2→D1), the transmission leakage (referenced LB) from the network (cellular) transmission severely impacts the D2D transmissions. This is seen in the signal constellation (below in FIG. 6B) where the QPSK points are blurred. The extra noise introduced in the transmitter will make the D2D transmission much more sensitive to interference in D2D reception (RX) at D1, implying a lower D2D performance etc. Since the network scheduler does not have all information about the D2D communication, for instance the distance between D2D and the amount of data transmitted between the devices (and hence transmission power needed), it is hard for the network to detect such transmission (TX) imbalance scenarios.
A simple prior art method to solve this problem is to always avoid scheduling (UL and/or DL) resources to a radio frequency communication device in a same sub frame as ongoing D2D communication. However, as mentioned above, such approach will significantly reduce spectrum usage and spectrum capacity, since in the existing approach only a subset of the frequency resources are utilized within a subframe.
Therefore, there is a need for a method and a radio frequency communication device that takes care of problems as described above without wasting resources.